If we encrypt the same piece of text using the monoalphabetic substitution cipher
and the Vigenère Cipher, we can see why the latter cipher is so much stronger
than the former. Let us use a short text about Vigenère to see the difference.
Firstly, you can see that the frequency distribution of letters in this messages
is fairly typical, with E being the most common letter:

Aged twenty six, Vigènere was sent to Rome on a diplomatic mission. It was here
that he became acquainted with the writings of Alberti, Trithemius and Porta,
and his interest in cryptography was ignited. For many years, cryptography was
nothing more than a tool that helped him his diplomatic work, but at the age of
thirty nine, Vigènere decided that he had amassed enough money to be able
to abandon his career and concentrate on a life of study. It was only then that
he began research into a new cipher.

Now, if you encrypt this message using a monoalphabetic substitution cipher, you can see how the frequency distribution changes:

The high frequencies have merely moved to different letters (e.g., the highest peak has moved from E to D, because E has been encrypted as D), and they can be used to crack the cipher. Now, let us encrypt the text using the Vigenère cipher (Keyword CHARLESV) and you will see why it is a better cipher:

As you can see, the frequency distribution is now much flatter. The peaks are less obvious, because each letter has been encrypted in 8 different ways, because the keyword is 8 letters long. The peak that was at E has been shared among 8 other letters. A flatter frequency distriibution means a much stronger cipher.